Bug 106269

What does your /etc/sysconfig/network-scripts/ifcfg-* and /etc/sysconfig/network
look like in the case that doesn't work for you?

I modified the IPs and snipped the MAC addresses (on general principle). You can
assume the 192.168.1.0/24 network is a public IP network (i.e. non-RFC1918). The
below is the non-working version; adding "GATEWAYDEV=eth0" to
/etc/sysconfig/network makes it work.

::::::::::::::
/etc/sysconfig/network
::::::::::::::
NETWORKING=yes
HOSTNAME=lamb.example.com
GATEWAY=192.168.1.1
::::::::::::::
/etc/sysconfig/network-scripts/ifcfg-eth0
::::::::::::::
# Intel Corp.|82801BA/BAM/CA/CAM Ethernet Controller
DEVICE=eth0
BOOTPROTO=none
BROADCAST=192.168.1.255
HWADDR=[snipped]
IPADDR=192.168.1.12
NETMASK=255.255.255.0
NETWORK=192.168.1.0
ONBOOT=yes
TYPE=Ethernet
USERCTL=no
PEERDNS=no
GATEWAY=192.168.1.1
::::::::::::::
/etc/sysconfig/network-scripts/ifcfg-eth1
::::::::::::::
# Macronix, Inc. [MXIC]|MX987x5
DEVICE=eth1
BOOTPROTO=none
HWADDR=[snipped]
ONBOOT=yes
TYPE=Ethernet
USERCTL=no
PEERDNS=no
NETMASK=255.255.255.0
IPADDR=172.20.100.21
GATEWAY=172.20.100.1

So, it sets the default route via 192.168.1.1, yet on device eth1? Or it makes
172.20.100.1 the default route?

It sets the default route to 172.20.100.1.

As best I've been able to tell from browsing the relevant bits of code, what
happens is that on the interface last brought up it will forcibly remove any
existing default route and add in a new one. I suspect it's this code from line
360 of "ifup":

-- cut
    # Set a default route.
    if [ -z "${GATEWAYDEV}" -o "${GATEWAYDEV}" = "${REALDEVICE}" ]; then
        # set up default gateway. replace if one already exists
        if [ -n "${GATEWAY}" -a "`ipcalc --network ${GATEWAY} ${NETMASK}
2>/dev/null`" = "NETWORK=${NETWORK}" ]; then
            ip route replace default via ${GATEWAY} ${WINDOW:+window $WINDOW} ${SRC}
        elif [ "${GATEWAYDEV}" = "${DEVICE}" ]; then
            ip route replace default ${SRC} ${WINDOW:+window $WINDOW} dev
${REALDEVICE}
        fi
    fi
-- cut

But I'm afraid I speak shell too little to be sure I grok this.

IIRC, the problem is that the GATEWAY in ifcfg-* will override the GATEWAY in
/etc/sysconfig/network. So you don't know that there's a global preference
without GATEWAYDEV.

The documentation in sysconfig.txt has been clarified a little in CVS
for future builds. The behavior you're seeing is pretty much what's
expected in the absence of GATEWAYDEV, though.